It is well known that fuel combustion yields emissions of pollutants such as unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). In order to improve air quality, practices are commonly taken to lower the emissions of such pollutants by filtering combustion engine exhaust through an exhaust treatment system that is inclusive of a substrate loaded with a catalytic material that is active to at least partially purify the exhaust gases.
The substrates on which catalytic materials are loaded are often cylindrical support bodies, having two end faces, a circumferential surface and an axial length L, and which are traversed from the first end face to the second end face by a multiplicity of channels. Such support bodies are often referred to as honeycomb bodies, may take the form of flow-through honeycomb bodies or wall-flow honeycomb bodies, and may be made of different ceramic or metallic materials such as cordierite, silicon carbide, steel and the like.
The substrates are loaded with the catalytic material applied in the channels of the support body, the application of which may be achieved by several different methods. A common coating method is the use of a catalyst slurry in which one or more catalytic components are suspended in a liquid slurry component (i.e., a washcoat). There are a number of known processes for depositing such a catalyst slurry onto the substrate. For example, the substrate may be immersed in the catalyst slurry or the catalyst slurry may be poured over the substrate. Currently preferred coating methods include aligning the substrate along a vertical axis, with one end face positioned as a bottom end face and the other end face positioned as a top end face, and introducing the catalyst slurry through either the top or bottom end face by application of a pressure differential that promotes a pushing or pulling of the catalyst slurry into the channels of the support body to thereby coat the channels.
It is important the coating concentration of a given layer of catalytic material (i.e., a catalyst layer) be at least equal to a “target uptake”, which is understood as the coating concentration in the wet-state that is required for achieving a target catalytic activity in a dry-state. When a coating process includes the application of two or more catalyst layers, it must be understood that there is a target uptake for each individual catalyst layer, as well as a target uptake for the entire coating process. In order to reduce costs, it is preferable the coating concentration of each catalyst layer (and the coating process as a whole) not significantly overshoot the respective target take-up. It is also important that each catalyst layer be applied in a uniform manner, in both the radial and axial directions of the substrate, as irregularities in a catalyst layer may lead to performance problems such as flow restriction and/or inadequate purification of the exhaust gases.
There are a number of different processes that may be used for applying a catalyst layer with a suitable coating concentration and uniformity. In one coating process, such as a flood-fill coating process, an amount of the catalyst slurry that is previously determined to exceed the target uptake is introduced into the channels of the substrate under the force of a pressure differential. This excessive amount may be an amount that exceeds an amount previously determined to correspond with the target uptake; may be a filling amount that is predetermined to correspond with the empty volume of the channels being filled, either in their entirety or to a predetermined height; or may simply be such an amount that is reached by continually introducing the catalyst slurry from one end face until the slurry is determined to have reached or emerged from the opposite end face. Following introduction of the excessive amount of catalyst slurry, there is then performed a purging of the substrate whereby a sufficient amount of the excess catalyst slurry is removed from the substrate under the force of a pressure differential (e.g., by suction or blowout) to thereby open the channels while leaving a catalyst layer along the channels. Thereafter, steps may be taken to introduce or remove further corrective quantities of the catalyst slurry to account for any undershooting or overshooting, so as to adjust the coating concentration to be within an acceptable tolerance of the respective target uptake.
In another coating process, such as a zone coating process, an amount of the catalyst slurry that is previously determined to correspond with the target uptake is introduced into the channels of the substrate under the force of a pressure differential. In this process, since the amount of catalyst slurry is predetermined to correspond with the respective target uptake, the catalyst slurry does not entirely fill the channels, but is instead simply forced to flow along the channel walls to thereby coat the channels. Since the catalyst slurry does not fill the channels, this process does not require a pressure-based purge of the flow channels, though steps may be taken to remove or introduce further corrective quantities of the catalyst slurry to account for any undershooting or overshooting, so as to adjust the coating concentration to be within an acceptable tolerance of the target uptake.
Once a substrate is suitably loaded with the catalyst slurry, the substrate is then dried to evaporate the liquid component of the washcoat and solidify the catalyst layer, and subjected to at least one heat treatment (e.g., calcination) to further fix the layers and activate the catalytic components.
In response to increasingly stringent emissions standards, it has become a common practice to load a substrate with multiple catalytic materials of different though complementing compositions, with the different catalytic materials applied as separate catalyst layers within the channels of the support body. See, for example, U.S. Pat. No. 8,906,330 (Hilgendorff, et al.). In some instances, the catalyst layers are superposed one over another, with the separate layers overlapping either partially or in their entireties. By carefully controlling the formation of the different catalyst layers, there can be produced multilayered catalytic substrates that are specially adapted for performing specific catalytic processes with specific reaction sequences.
However, the loading of a substrate with multiple catalyst layers necessarily results in an increase in production time per loaded substrate, and a decrease in overall production throughput. An example of this is presented in U.S. Pat. No. 8,906,330, wherein the production of a catalytic substrate loaded with three catalyst layers (i.e., an inner layer, a middle layer, and an outer layer) includes the performance of a drying step and a calcining step after the application of each individual layer. Such a coating process is inefficient in that performance of multiple drying and calcining steps significantly increases production time for a single catalytic substrate. Indeed, whereas a catalyst layer can normally be applied as quickly as a few seconds, the performance of separate drying and calcining steps after the formation of each individual layer in the coating process of U.S. Pat. No. 8,906,330 necessitates approximately an additional hour of production time per catalyst layer—which, in the three layer example of U.S. Pat. No. 8,906,330, results in an approximate 200% increase in production time over that which would otherwise be possible if the drying and a calcining steps were performed only after application of all three layers.
Despite the inefficiencies associated therewith, as may be appreciated from the foregoing discussion of U.S. Pat. No. 8,906,330, it has been the conventional understanding that when applying multiple layers of catalytic materials a given layer must first be applied, dried, and calcined before a subsequent layer may be applied. Though not intending to be bound by theory, it is considered that the performance of such intermediate drying and calcining steps has been deemed necessary in the art as the introduction of a subsequent catalyst slurry to a substrate that is already loaded with an non-dried, still-wet catalyst layer has been found to result in the still-wet catalyst layer being partially removed and/or shifted due to forces generated by the pressure differential that is used for introducing the subsequent catalyst slurry. For example, if employing a pressure differential that generates a top-down application of a second catalyst slurry through a top end face of the substrate, then some quantity of a prior applied and still-wet first catalyst layer might be removed from the substrate through the bottom end face; might be shifted into a lower region of the substrate where the first catalyst layer is not intended for application; and/or might be redistributed in a non-uniform manner (e.g., with a gradient such that there is a greater thickness toward the bottom end face and a lesser thickness toward the top end face).
In addition, when applying a subsequent catalyst slurry that is of a different catalytic composition than the prior-applied and still wet catalyst layer, there is a further concern that when the subsequent catalyst slurry is introduced in a manner to overlap the still-wet catalyst layer the wet-on-wet interface of the two layers may promote a comingling of the different catalytic components of the separate layers, thereby contaminating and degrading the catalytic function of one or both layers.
Recently, attempts have been made to reduce the number of drying and/or calcining steps in a coating process. For example, U.S. Pat. No. 9,144,796 (Bennett, et al.) discloses a coating process for applying multiple catalyst layers to a substrate, wherein individual layers are applied in two-steps, with a first portion of a given catalyst slurry introduced from one end face such that the substrate is “part-coated” with a portion of a first catalyst layer, followed by flipping the substrate and subsequently introducing a second portion of that same catalyst slurry from the opposite end face to finish the first catalyst layer. U.S. Pat. No. 9,144,796 proposes reducing the number of steps in such a coating process by foregoing the performance of a drying step between application of the first “part-coated” portion of the first catalyst layer and the subsequent completion of the first catalyst layer though the opposite end face. In particular, U.S. Pat. No. 9,144,796 teaches that the drying step may be omitted between introduction of the two portions of the same catalyst slurry by formulating the catalyst slurry with a rheological modifier that enhances the viscosity of the slurry such that the first “part-coated” portion of a catalyst layer is not unduly disturbed upon introduction of the second portion of the same catalyst slurry through the opposite end face.
However, the coating process disclosed in U.S. Pat. No. 9,144,796 is inefficient in that it continues to require performance of a drying step between introduction of a first catalyst slurry (as applied in two portions) and introduction of a subsequent catalyst slurry which may have a different catalytic composition relative to that of the first catalytic slurry. Though not being bound by theory, it is considered U.S. Pat. No. 9,144,796 continues to require a drying step between introduction of the two separate catalyst slurries in order to forego a wet-on-wet interface between the two different catalyst layers, thereby avoiding the risk of comingling different catalytic components of the two layers, and the potential contamination and degradation of one or both layers.
Accordingly, there remains a need in the art for a method of coating a substrate with multiple catalyst layers that reduces the production time per loaded substrate, thereby permitting a greater overall production throughput, and which also reduces the risk of contamination between two superposed catalyst layers of different compositions.